Some good suggestions, mine would be:
Six Feet Under The finale sequence definitely, I think possibly the best piece of television I’ve ever seen. Buckets of tears and has continued to haunt me ever since.
Futurama The time skip episode and they’re watching the news. “Next we’ll see if new pop sensation Wendy (skip) won three Emmies last night and was later (skip) found dead in her bathtub”.
Babylon 5 The scene where Delenn goes to speak to the Grey Council to chew them out for not taking any action about the shadows and saying that if the warriors won’t fight then the other castes will, then storming out of the room followed by six of the Council members in silence. (Something I always liked about B5 was how for several series in the credits they had for Vir the image of him smiling and making a little wave which one would assume was the show making him out to be all fun and cuddly but is actually him telling Morton how he can’t wait for the day when he gets to see his head on a spike and wave to it as a reminder that some things come with too high a price.)
Deep Space Nine “Way of the Warrior” when the Klingons attack the station. Quark is intending to defend his bar and says to Odo “I’m going to defend my bar, with this” opening the box in his hands. Odo takes out the piece of paper inside and reads “Dear Quark I took the parts from your disrupter to fix the replicators, will replace them soon love Rom”. Quark says “I will kill him!” and Odo says “With what?”.
Voyager Scorpion part 1 where Voyager is attempting to talk to the Borg to tell them they have a weapon to fight species 8472. After the Borg realise what they are being offered they simply transport Janeway onto their ship and say “State your demands”. Diplomacy as only the Borg could do it.
Star Trek TNG When Data makes a child and she starts to malfunction. He has the final conversation with her before she breaks down and she says “I love you father” and he says “I wish I was able to respond”. I’ve watched that episode three times and each time I’ve cried my eyes out.
Drawn Together The gay marriage episode where Clara storms into the church and cries out “Stop, stop this abomination! Don’t you see? If the gays can get married then the institution of marriage will be destroyed. Society will crumble, rivers will run red with blood, and Nazis will walk the earth riding dinosaurs!”.
X Files The one where the killer projects his thoughts onto photos, at the start of the episode the victim has just been attacked by him after having her photo taken and the photographer looks at it to see a whole world of weirdness staring back at him.
Battlestar Galactica Just generally the whole thing with Number 6 appearing to Baltar and talking to him, I always love those bits.